1. Field of the Invention
This invention relates to communication systems, and more particularly to a method and apparatus for reducing co-channel interference in a frame-synchronized wireless communication system.
2. Description of Related Art
A wireless communication system facilitates two-way communication between a plurality of subscriber radio stations or subscriber units (either fixed or portable) and a fixed network infrastructure. Exemplary systems include mobile cellular telephone systems, personal communication systems (PCS), and cordless telephones. The objective of these wireless communication systems is to provide communication channels on demand between the subscriber units and the base station in order to connect the subscriber unit user with the fixed network infrastructure (usually a wired-line system). In the wireless systems using multiple access schemes, frames of time are the basic transmission unit. Each frame is divided into a plurality of slots of time. Some time slots are used for control purposes and some time slots are used for information transfer. Information is typically transmitted during time slots in the frame where the time slots are assigned to a specific subscriber unit. Subscriber units typically communicate with the base station using a “duplexing” scheme that allows for the exchange of information in both directions of connection.
Transmissions from the base station to the subscriber unit are commonly referred to as “downlink” transmissions. Transmissions from the subscriber unit to the base station are commonly referred to as “uplink” transmissions. Depending upon the design criteria of a given system, the prior art wireless communication systems have typically used either time division duplexing (TDD) or frequency division duplexing (FDD) methods to facilitate the exchange of information between the base station and the subscriber units. Both the TDD and FDD duplexing schemes are well known in the art. Exemplary wireless communication systems using these schemes are described in more detail in the related U.S. Pat. No. 6,038,455, by Gardner et al., issued Mar. 14, 2000, entitled “Reverse Channel Reuse Scheme in a Time Shared Cellular Communication System”, which has been incorporated by reference herein for its teachings on wireless communication systems.
Some communication systems do not use time frames in communicating between the base station and their respective and associated subscriber units (or “terminal stations” in Broadband Wireless Access (BWA) communication systems). For example, BWA systems based on cable modem technologies do not use time frames when communicating on either the uplink or the downlink. Therefore, these systems do not allow for frame synchronization between base stations and disadvantageously do not permit coordination between the base stations for purposes of reducing co-channel interference. Similarly, un-synchronized TDD systems allow different communication cells within the system to be “free running”, in that different cells and sectors within the system operate on frames that are not synchronized in time.
Wireless communication systems rely upon frequency re-use because frequency allocation or bandwidth is typically limited. For example, in cellular communication systems and broadband wireless systems, geographic areas or regions are typically divided into cells that are nominally hexagonally or square shaped. As described in U.S. Pat. No. 6,038,455, each cell or sector is allocated one or more radio frequency channels. For example, in a cellular communication system utilizing frequency division multiple access (FDMA), adjacent or nearby cells are assigned separate frequencies. After all available frequencies have been allocated, it is necessary to begin reusing the frequencies. For example, if four frequencies are available, it is necessary to begin using the first frequency again starting in the fifth cell. Due to the nature of the systems described in the incorporated U.S. Pat. No. 6,038,455, and in PCS, cellular and paging systems of the prior art, frequency re-use cannot be used as aggressively as it can be used in BWA systems. For example, in PCS/cellular/paging systems, typically only a fraction of the frequency spectrum is used per cell. In contrast, in BWA, frequency re-use can be much more aggressive (for example, a frequency can be re-used at least once per cell, with multiple sectors).
FIG. 1a is a simplified diagram of an exemplary broadband wireless configuration showing frequency re-use. In broadband wireless communications, a plurality of base stations 1 communicate with fixed terminal stations.(i.e., “subscriber units”). As shown in FIG. 1a, clusters of four sectors 4 surrounding base stations 1 (1a-1d) form cells 2 (2a-2d). The cells 2 are shown as being separated by the bold lines 30 and 32. In BWA systems, a cell 2 typically comprises either four or six sectors 4. In the case of four sectors 4, the coverage area covered by the cell is square (as shown in FIG. 1a). In the case of six sectors, the coverage area covered by the cell is hexagonal (as exemplified in FIG. 6 described below).
Thus, each cell 2 has an associated and corresponding base station 1. For example, cell 2a has an associated and corresponding base station 1a. Cell 2b has an associated and corresponding base station 1b, and so on. Each base station 1 typically includes an array of sectored antennas for communicating with the terminal stations within the cells 2. In accordance with broadband wireless technology, a sectored antenna is typically 60 or 90 degrees in beamwidth for commnunicating with terminal stations within an entire sector. Thus, in a four-sector case, a base station 1a comprises at least four sectored antennas, one antenna per sector 4 (4a-4d). In a six-sector case, the base station comprises six sectored antennas. Each sector contains a plurality of terminal stations that communicate with the base station 1a on a unique radio frequency (RF) channel.
In broadband wireless systems, each terminal station utilizes a highly directional antenna (typically less than 3 degrees beamwidth) for communicating with its associated base station 1. The highly directional antenna is fixed and pointed toward the associated base station 1. The base station's sectored antenna receives energy from any terminal station operating on the same RF channel and is positioned on a line of sight relative to the sectored antenna. Line of sight (LoS) is defined herein as an unobstructed (first Fresnel zone clear) radio wave propagation path between a transmitting antenna and a receiving antenna. On the downlink, a base station's 1 sectored antenna transmits energy on an RF channel to a terminal station's highly directional antenna. On the uplink, a terminal station's highly directional antenna transmits energy on an RF channel to a base station's 1 sectored antenna.
In accordance with frequency re-use methodologies and techniques, a set of RF channels is allocated for use in each cell 2 (for example, cells 2a, 2b, 2c and 2d). As shown in FIG. 1a, for example, each cell 2 utilizes a set of four orthogonal RF channels (A, A′, B, and B′) comprising two frequencies (A and B) wherein each frequency has two different polarizations (designated by the “non-primed” and “primed” indicators). Each sector 4 (4a-4d) of a cell 2 therefore utilizes a different orthogonal RF channel for communication between terminals in the sector and an associated sector base station (i.e., a terminal in sector 4a uses frequency A, a terminal in sector 46 uses frequency B′, a terminal in sector 4c uses frequency B, and a terminal in sector 4d uses frequency A′). The set of four orthogonal RF channels is then reused as shown in FIG. 1a in adjacent cells 2 (for example, in cells 2b, 2c and 2d). As shown in FIG. 1a, in each cell 2 (i.e., in cells 2a, 2b, 2c and 2d), the pattern of frequency distribution is normally a mirror image of adjacent and diagonal cells 2. Thus, the upper left-hand sector 4a of the upper left-hand cell 2a uses the same frequency (e.g., frequency A) as the upper right-hand sector 4b of the adjacent cell 2b. It also uses the same frequency as the lower right-hand sector of cell 2d, and the lower left-hand sector of cell 2c.
Because frequencies are re-used, two cells or sectors operating on the same usable frequency, though separated geographically, may interfere with each other. This is known as “co-channel interference”. The effect of co-channel interference varies with terrain and distance. In cases where path loss conditions favor the desired signal, the co-channel interference may not be strong enough to have a significant impact on receiver performance. In other cases, path loss conditions may cause the difference between the desired carrier power and the interference (known as the “C/I” ratio) to be insufficient for good receiver performance. Co-channel interference is inversely proportional to a wireless communication system's capacity (i.e., ability to communicate with multiple terminal stations). Thus, as co-channel interference increases, system capacity decreases.
Well-known modulation schemes such as quadrature amplitude modulation (QAM) and quadrature phase shift keying (QPSK) are utilized in broadband wireless communications for efficiently transmitting data between terminal stations and base stations. Three typical modulation schemes for use in broadband wireless communication systems are QAM-64, QAM-16 and QPSK. When the C/I ratio is low (i.e., relatively high co-channel interference) a robust modulation scheme must be used. QPSK is a robust modulation scheme that can operate at a C/I ratio of approximately 11 dB or greater.
FIG. 1b shows a simplified graphical representation of an exemplary broadband wireless frequency re-use configuration showing the potential problems associated with co-channel interference. As shown in FIG. 1b, terminal stations located within sectors T1-T4 operate on an RF channel (A). A terminal station located within sector T1 can receive unwanted radio frequency energy from base stations 1e, 1f and 1g because these base stations 1e-g operate on the same RF channel (A) and could be in the LoS of the terminal station. Similarly, base station 1g can receive unwanted radio frequency energy from terminal stations located within sectors T1-T3. Receiving unwanted radio frequency energy causes co-channel interference.
In the exemplary broadband wireless communication system of FIG. 1b, the uplink is less spectrum efficient than is the downlink. The uplink spectrum efficiency suffers because the base station collects energy using a sector antenna having a much greater beamwidth (typically 60 to 90 degrees) while the terminal station collects energy using a highly directional antenna (typically less than 3 degrees beamwidth). Thus, the downlink can support less robust modulation schemes (e.g., QAM-64 and QAM-16) while the uplink may require the use of more robust modulation schemes (e.g., QPSK).
FIG. 2 shows a graphical representation of the uplink C/I ratio of base station 1g of FIG. 1b versus the probability of line of sight (LoS) activity of potentially interfering terminal stations (i.e., terminal stations located within sectors T1-T3). Only sectors in nearby cells are considered in this analysis because the LoS conditions and propagation characteristics change dramatically for distant cells (typically, 10-15 KM in millimeter wave frequencies). The vertical axis represents the C/I ratio measured in dB. The horizontal axis represents the probability (measured as a percentage) of a terminal station located outside of sector T4 having a LoS position relative to the base station 1g. The plot line labeled “T1+T2+T3” represents potentially interfering terminal stations from sectors T1-T3. The plot line labeled “T1 only” represents potentially interfering terminal stations from only sector T1. Co-channel interference is greatest (i.e., worst case scenario occurs) when the probability of LoS is 100 percent. Thus, the C/I ratio reaches its smallest value (i.e., co-channel interference is at its greatest) when the probability of LoS is 100%.
Referring to the T1+T2+T3 plot line of FIG. 2, when the probability of LoS of all potential interfering terminal stations located within sectors T1, T2 and T3 is greater than 50%, the C/I falls below 11 dB. Thus, the communication system's capacity is greatly reduced because not even the most robust modulation scheme (i.e., QPSK) can operate when the C/I falls below 11 dB. Referring to the “T1 only” plot line of FIG. 2, when the probability of LoS of all potential interfering terminal stations located within sector T1 is 100%, the C/I remains above 14 dB. Thus, the communication system's capacity is not as greatly affected as it is in the previous scenario because the most robust modulation scheme (i.e., QPSK) can operate when the C/I remains above 11 dB. However, at 100% LoS conditions, the more robust scheme is required. This increases the costs associated with deployment of the system and reduces the efficiencies. Therefore, eliminating co-channel interference from selected sectors (e.g., T2 and T3) will greatly increase system capacity.
Therefore, a need exists for a method and apparatus for reducing co-channel interference in a wireless communication system. The co-channel interference reducing method and apparatus should increase the wireless communication system's capacity. Such a co-channel interference reducing method and apparatus should utilize uplink and downlink bandwidth in an allocation-efficient manner. The present invention provides such a co-channel interference reducing method and apparatus.